Cylinder oil for lubricating between the cylinder and piston and system oil responsible for lubricating and cooling other parts are used in crosshead diesel engines (see Patent Literatures (PTL) 1 to 6 listed below). System oil for crosshead diesel engines for ships is provided to the piston undercrown to cool the piston, yet since the piston undercrown is at a high temperature, heat exchange efficiency lowers if sludge or the like accumulates, and damage occurs to the piston due to heat (piston fracture). System oil for crosshead diesel engines for ships does not come into direct contact with combustion gas in the combustion chamber unlike other engine oils, and so can be regarded as a kind of hydraulic oil. If drips of the cylinder oil mix into and contaminate the system oil, thermal stability worsens, which makes coking occur more easily and may lead to sludge accumulating on the piston cooling surface. Therefore, high temperature detergency and an anti-coking property are important in system oil for crosshead diesel engines.
The base oils used in conventional lubricating oil are mainly manufactured by separating gasoline or a gas oil component from crude oil by distillation, further subjecting the resulting atmospheric distillation residue to reduced-pressure distillation, bringing out the necessary viscosity fraction, and refining the result. These base oils are classified in group I under the API base oil categorization.
Since the sulfur content and the aromatic content included in the base oil adversely affect the oxidation stability of the base oil, the above-mentioned residue has in recent years started to be subjected to hydrocracking to manufacture a base oil with an extremely small sulfur content and aromatic content. Wax manufactured by the Fischer-Tropsch process, petroleum-based wax which is a by-product when manufacturing base oil, or the like, is also subjected to hydrocracking to manufacture base oils with an extremely high viscosity index. These base oils manufactured by hydrocracking are classified in group II or III under the API base oil categorization.
During the refining process of the former base oils (group I), many processes are used to employ a solvent such as furfural, phenol, methylpyrrolidone, or the like to selectively extract and remove unstable compounds centering on the aromatic content. By contrast, in the manufacturing method of the latter base oils, the aromatic content in the base oils is extremely low, and there is nearly no need to undergo the above-described solvent refining processes. Therefore, the relative amount of base oil manufactured by undergoing a solvent refining process (i.e. group I base oil) is declining.